1. Technical Field
The present invention relates in general to a CRT display, and in particular to a computer system comprising such a CRT display, together with a processor connected to the CRT display for generating raster synchronization signals and for varying the frequency of at least one of the raster synchronization signals to produce different display modes. More particularly, the present invention relates to a line scan circuit for a CRT display used in such a computer system.
2. Description of the Related Art
The present invention relates to deflection apparatus for a raster scanned CRT display to maintain a constant picture size despite changes in raster line scanning frequency.
Many raster scanned CRT displays are now designed to be compatible with a wide range of computer systems each capable of generating one or more different raster display formats. Each raster display format is generally characterized by a different pair of line and frame scanning frequencies.
A conventional line scan circuit for driving the horizontal deflection coils for a CRT display comprises a ramp generator for producing a line scan current signal in the deflection coils in synchronization with a raster line synchronization (sync) signal generated by the host computer. The line scan signal is generally in the form of a sawtooth waveform. However, to bring the angular velocity into line with the arc of the CRT screen to ensure that the or each electron beam tracks each line of the raster with constant velocity, a degree of "S correction" is applied to the line scan signal. The degree of "S correction is determined by an S correction capacitance conventionally connected in series with the deflection coils.
The amplitude of the line scan signal is inversely proportional to the frequency of the line sync signal. Therefore, the width of the picture displayed on the CRT screen is inversely proportional to the frequency of the line sync signal.
Typically, in CRT displays that are capable of displaying different raster display formats, a ramp regulator is connected to the ramp generator to reduce the effect of changes in line sync frequency on displayed picture width by varying the amplitude of the line scan signal as a function of a reference input and a feedback signal. The feedback signal is generated as a function of the line scan signal by a feedback circuit connected to the ramp generator and the regulator.
In some conventional CRT displays, the ramp regulator is configured to operate only within discrete bands of line sync frequencies each centered on a different commonly-used line sync frequency. Each band is separated from adjacent bands by a dead band. The dead bands prevent the ramp regulator from spuriously toggling between adjacent bands.
A disadvantage associated with displays having banded width regulation is that they cannot operate at line sync frequencies within the dead bands. Furthermore, complex switching circuitry is required to implement banded width regulation. Thus, displays having banded width regulation contain a large number of components and are consequently expensive to manufacture.